Werner's Theory of Coordination Compounds Video Lecture - Class 12

have you ever wondered why a stable salt
such as cobalt trichloride
combined with a group of stable
independently existing molecules such as
ammonia to form new compounds like cocl
3 6 NH 3 cocl 3 5 NH 3 and cocl 3 4 NH 3
with entirely new properties
Alfred Werner
a Swiss chemist
after preparing thousands of new
compounds and studying their properties
put forward a theory to explain the
formation of complex compounds
the first successful explanation for the
phenomenon
the theory became famous as the
coordination theory of complex compounds
also known as oneness theory
his explanation of complex formation is
based on some postulates
the first postulate is that the central
metal atom or ion in a coordination
compound exhibits two types of valances
primary and secondary
while writing the structure of the metal
complexes the ligands that are linked by
primary valency are shown by dotted
lines while those linked by a secondary
valency are shown by pink lines if a
ligand satisfies both the valence EES
then it is shown by a thick as well as
by a dotted line and it is set to
exhibit dual character
the second postulate is that dry memory
balances are ionizable
and correspond to the number of charges
on the complex ion
primary balances apply equally when two
simple salts and two complexes
and are satisfied by negative ions
for example
the primary valency of two in cocl2
is satisfied by two chloride ions
similarly
the primary valency of three in the
complex C or NH 3 6 CL 3 is satisfied by
three chloride ions in modern
terminology the primary valency
corresponds to the oxidation number
third postulate is that secondary
balances correspond to the balances that
a metal atom or ion exercises towards
neutral molecules or negative ions in
the formation of its complex ions
every metal has a fixed number of
secondary valence ease or coordination
number
example
cobalt 3 plus platinum 4 plus and iron 3
plus ions are recognized to have a
coordination number of 6
similarly nickel two plus
two plus and platinum two plus ions have
a coordination number of four
secondly valances are non ionizable
satisfied by neutral molecules or
negative ions
the secondary valency is equal to the
coordination number of the metal
for example in the complex cocl three
six in each three the three chlorides
are held by primary balances and the six
ammonia ligands are held by secondary
balance seeds
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the fourth postulate is that secondary
balances are directional
so the complex has a particular shape
the number and arrangement of ligands in
space determines the stereochemistry of
a complex
the most common coordination number in
transition metal complexes is six
and the shape is usually octahedral
the coordination number four is also
common
and this gives rise to either
tetrahedral or square clay now complexes
this postulate predicted the existence
of ice for medicine in coordination
compounds
to distinguish between the two types of
balances Warner introduced the concept
of using square brackets to enclose the
atoms that make up a coordination
complex and which are therefore
not ionized
the postulates of Werner's coordination
theory were actually based on
experimental evidence rather than
theoretical
weren't treated cold solutions of a
series of compounds of cobalt 3 chloride
with ammonia with an excess of silver
nitrate
and wait the silver chloride
precipitated
the stoichiometries of the complex
silver chloride formed well as shown
here
one deduced that in cuc and three six in
each three
the primary balance II or oxidation
state of +3 is satisfied by three
chloride ions
the coordination number of the cobalt 3
plus ion is 6
as there are six ammonia molecules in
the compound they alone satisfy the six
secondary valence ease of Cobalt
as you can see
the primary balances are represented by
dotted lines and the secondary balances
by thick lines
you
in modern terms
the complex is written as CEO nh3 6 CN 3
the three chloride ions are ionic
are precipitated as silver chloride by
silver nitrate
the complex will ionize in solution and
give four ions
that is
1c o n 8 3 6 3 + ion and 3 chloride ions
miRNA deduced that the loss of one
ammonia molecule from COC and three six
nh3 should give COC and 3 5 NH 3
this complex has only five ammonia
molecules
therefore
satisfy the coordination number of six
of cobalt
one chloride ion assumes dual behavior
that is
it satisfies the primary as well as the
secondary valency of Cobalt
in modern terms
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the complex can be written as Co nh3 v
CN cn 2
does
only two of the three player ions are
ionic
only two are precipitated as silver
chloride with silver nitrate
oh no you nice even
this complex will give three ions
one co nh3 5 CL 2 plus ion and two
chloride ions
as can be seen from its structure
one chloride ion
which is playing a dual role
is shown by a pic as well as the dotted
line
similarly
in cocl 3 ford nh3
satisfy the secondary valency of six
to chloride ions assume dual behavior
in modern terms
complex can be written as co nh3 for CL
2 CL
dance only one your ID ion can be
precipitated as silver chloride
this complex
ionization gives two ions
one of co nh3 for CL 2 plus and the
other of chloride ion
the structure of the complex is shown
here
although awareness theory successfully
explains the bonding features in
coordination compounds
it suffers from some drawbacks
doesn't explain why only certain
elements form coordination compounds
does not explain why the bonds in
coordination compounds have directional
properties
it does not explain the color
and the magnetic and optical properties
of complexes
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